Nocturia and sleep disruption are often associated in the literature. This association is almost exclusively mentioned in the direction nocturia leads to sleep disruption. However, few authors have implied that the opposite is probably also true: sleep disruption could explain the complaint of nocturia in healthy older people. Indeed, up to a third of older people complain of early morning awakening and sleep maintenance insomnia, sleep problems rarely seen in young adults. These age-related sleep disorders are associated with a misalignment between the timing of the sleep-wake cycle and the phase of the circadian system and diminished amplitude of the circadian rhythms of melatonin and core body temperature. The misalignment has a profound negative impact on sleep consolidation and sleep efficiency in older people. On the other side, the origins of nocturia can be categorized into three main mechanisms: increased urinary output at night, diminished bladder capacity or a combination of both. From this we can define two main mechanisms by which the aging of the circadian pacemaker and its related sleep disruption could explain nocturia: 1) Since healthy older people have a less consolidated and shallower sleep, they are more likely than younger people to awake to arousal signals such as noise or bladder stretch signal and then micturate. 2) Healthy older people produce a significantly higher proportion of their urine at night compared to younger people. This could result from the blunting of the circadian rhythms of diuretic or anti-diuretic hormones. The proposed investigation will address the following Specific Aims: Specific aim 1 Test the hypothesis that healthy older people have more awakenings with nocturnal micturition than younger people in a night with normal sleep pressure. Specific aim 2: Test the hypothesis that during a night with high sleep pressure, older people will experience fewer awakenings with urine voiding compared to a night with normal sleep pressure. Specific aim 3: Test the hypothesis that the known increase in nocturnal urinary output in healthy older people is due to the attenuated circadian variations of the following anti-diuretic and diuretic hormones vasopressin, plasma renin angiotensin, aldosterone, and atrial natriuretic peptide when studied in strictly controlled conditions of posture, food intake, lighting and wake. Up to 75% of older people have a complaint of nocturia and this has long-term morbidity and mortality implications. So understanding more precisely its pathophysiology can have important implications for public health in the long term.